I have a question about using a valve on the siphon outlet. If you were siphoning with the vent closed and the water level lower than the vent opening, what would happen if you closed the outlet valve? Assuming your fittings were air tight would you be able to interrupt the siphoning and come back to it days later without needing to re-prime the pipe?
There was a point when I wasn't sure if the emergency spillway was going to hold and I thought it would be wise to partially drain the pond. Water was already going over the ES and more heavy rains where in the forecast. If I drained it several feet and then broke the prime, then I couldn't have easily restarted the siphon until the pond refilled. If it did rain heavily it would have been nice to be able to begin siphoning before the pond filled.
